Studies show that men with low testosterone are more likely to develop metabolic syndrome. In short-term studies, testosterone replacement improved blood sugar levels and obesity in men with low testosterone. The long-range benefits and risks are still unknown.

Testosterone and Heart Disease

Testosterone has mixed effects on the arteries. Many experts believe testosterone contributes to the higher rates of heart disease and high blood pressure that tend to affect men at younger ages. By this reasoning, high testosterone might be bad for the heart.

But testosterone deficiency is connected to insulin resistance, obesity, and diabetes. Each of these problems increases cardiovascular risk. Men with diabetes and low testosterone also have higher rates of atherosclerosis, or hardening of the arteries.

A certain amount of testosterone may be necessary for healthy arteries because it's converted into estrogen, which protects arteries from damage. As yet, no studies show that testosterone replacement protects the heart or prevents heart attacks.

Testosterone and Other Conditions

Low testosterone often exists with other medical conditions:

Depression: In a study of almost 4,000 men older than 70, those with the lowest testosterone levels were more than twice as likely to be depressed. This link remained even after allowing for age, general health, obesity, and other variables.

Erectile dysfunction (ED): Problems with erections are one of the most common symptoms of low testosterone. Most ED is caused by atherosclerosis. Men with risk factors for atherosclerosis -- diabetes, metabolic syndrome, or obesity -- often have low testosterone, too.

High blood pressure: The effects of testosterone on blood pressure are many and complex. Men with high blood pressure may be almost twice as likely to have low testosterone as men with normal blood pressure. On the other hand, too much testosterone can increase blood pressure. Testosterone acts in multiple ways on blood vessels, so this may account for the varying effects.